This invention relates to a power supply apparatus for use in electroplating, for supplying current to a load including an object to be plated, an electrolytic solution and electrodes, to thereby plate the object. (In this specification, this type of power supply apparatus is referred to as electroplating power supply apparatus, and the load is referred to as plating load.)
In electroplating, it is known to invert, at a high speed, the polarity of current supplied to a plating load. When current with the positive polarity is being supplied to the plating load, plating takes place, and when the current of the negative polarity is being supplied to the load, the plating is interrupted or part of the metal forming a plated layer is dissolved back into the electrolyte solution, whereby crystals forming the plated layer are made finer so that the object can be uniformly plated.
When a multi-layered printed circuit board 2, like the one shown in FIG. 1, is plated, some problems have happened. The multi-layered printed circuit board 2 includes substrates 2a and 2b, for example, on which electronic components are integrated to a high density. The circuit board 2 are provided with a number of through-holes like a through-hole 4, and a number of via-holes, like a via-hole 6. As the number of layered substrates is larger, the difference in thickness between plated metal layers on an edge 4E and an inner wall 4IN of the through-hole 4 becomes larger, resulting in non-uniform plating. In other words, it is difficult to uniformly plate the through-holes 4. Similarly, as the number of substrates increases, the difference in thickness between plated metal layers on an edge 6E and an inner wall 6IN of the via-hole 6 becomes larger, which results in non-uniform plating. It has been found that in order to form a plated metal layer uniform in thickness over the entire surfaces of the substrates 2a and 2b, it is necessary to make a negative-polarity plating current of sufficiently larger magnitude flow for a shorter time period than a positive-polarity plating current.
In Japanese Patent Application No. HEI 10-281954 on Sep. 17, 1998 (Japanese Patent Application Publication No. 2000-92841), inventors including one of the inventors of the present invention proposed an electroplating power supply apparatus which supplies current having a polarity inverted at intervals of, for example, from 5 to 20 milliseconds, to a plating load, to thereby form a layer of uniform thickness over a plating load including a plurality of substrates, such as a multi-layered printed circuit board. The apparatus of this Japanese patent application is shown in FIG. 2.
The power supply apparatus shown in FIG. 2 includes DC power supplies 10a and 10b, voltage-boosting converters 16a and 16b, and choppers 22a and 22b, for supplying a plating load 24 with current alternating between positive and negative polarities. The voltage-boosting converter 16a includes a reactor 12a and an IGBT 14a, and the voltage-boosting converter 16b includes a reactor 12b and an IGBT 14b. The chopper 22a includes a reverse current blocking diode 18a and an IGBT 20a, while the chopper 22b includes a reverse current blocking diode 18b and an IGBT 20b. The IGBTs 14a, 14b, 20a and 20b are controlled by a controller 26.
For example, when the IGBTs 20a and 14b are nonconductive and the IGBTs 20b and 14a are conductive, current flows through the DC supply 10a, the reactor 12a and the IGBT 14a, resulting in storage of energy in the reactor 12a. At the same time, a negative current is supplied to the plating load 24 from the DC supply 10b, through the reactor 12b, the diode 18b and the IGBT 20b. 
Then, the IGBTs 20a and 14b are rendered conductive and the IGBTs 20b and 14a are rendered nonconductive, a positive current flows from the DC supply 10a through the reactor 12a, the diode 18a and the IGBT 20a to the plating load 24 to plate an object to be plated. In this case, because of the turning off of the IGBT 14a, the voltage based on the energy stored in the reactor 12a is superposed on the voltage supplied by the DC power supply 10a, resulting in rapid change of the negative current flowing to the plating load 24 to the positive current. At the same time, energy is stored in the reactor 12b because the IGBT 14b is conductive.
Then, the IGBTs 20a and 14b become nonconductive again, with the IGBTs 20b and 14a rendered conductive, a negative current is supplied to the plating load 24 from the DC power supply 10b through the reactor 12b, the diode 18b and the IGBT 20b. In this case, too, the voltage generated in the reactor 12b is superposed on the voltage supplied by the DC power supply 10b, so that the change from the positive current to the negative current is rapid.
In this way, current with alternating polarity is supplied to the plating load 24, and a multi-layered printed circuit boards with through-holes and via-holes can be plated with a layer of a uniform thickness.
The power supply apparatus described above requires separate DC power supplies 10a and 10b for positive and negative currents. Also, it requires, in addition to the IGBTs 20a and 20b used to switch the main current, the auxiliary IGBTs 14a and 14b for inverting the load current at a high speed, which makes the circuit arrangement complicated, which, in turn, leads to increase of the cost of the electroplating power supply apparatuses.
In order to downsize the electroplating power supply apparatus, the DC power supplies 10a and 10b are downsized by arranging them to rectify a commercial AC signal, convert the resulting rectification output to a high-frequency signal in an inverter, and transform and rectify the high-frequency signal to a DC signal.
The value of commercial AC power supply voltage differ from country to country or from area to area. Therefore, in electroplating power supply apparatuses for use in countries or areas where AC commercial power supplies provide a xe2x80x9c400 V groupxe2x80x9d voltage, i.e. a voltage of from 380 V to 460 V, the DC power supplies 10a and 10b require an inverter including IGBTs which can withstand a voltage resulting from rectifying the xe2x80x9c400 V groupxe2x80x9d AC voltage. However, such IGBTs are not widely available, so they are expensive, leading to increase of the cost of the power supply apparatuses.
An object of the present invention is to provide an inexpensive power supply apparatus for use in electroplating, which can provide uniform electroplating.
An electroplating power supply apparatus according to the present invention includes an input-side rectifier for rectifying a commercial AC signal. The output signal of the input-side rectifier is converted to a high-frequency signal in a DC-to-high-frequency converter. A chopper or an inverter may be used as the DC-to-high-frequency converter. A plurality of DC-to-high-frequency converters may be used, being connected in series. When a plurality of DC-to-high-frequency converters are used, they are connected in series. The high-frequency signal outputted by the DC-to-high-frequency converter is transformed in a transformer. The number of transformers is equal to the number of the DC-to-high-frequency converters. When plural DC-to-high-frequency converters are used, the same number of transformers are used being connected in parallel.
A first output-side rectifier is connected between the transformer and a load to rectify a transformed high-frequency signal provided from the transformer in such a way that a positive polarity current can be supplied to the load when the transformed high-frequency signal is positive in polarity. A second output-side rectifier connected in parallel with the first output-side rectifier rectifies the transformed high-frequency signal of negative polarity so that a negative polarity current can be supplied to the load. The first and second output-side rectifiers are arranged to perform full-wave or half-wave rectification.
A first semiconductor switching device is connected in series with the first output-side rectifier and is ON-OFF controlled by a lower-frequency signal at a frequency lower than that of the high-frequency signal. A second semiconductor switching device is connected in series with the second output-side rectifier and is placed in the opposite state to that of the first semiconductor switching device in accordance with the lower-frequency signal. In other words, when the first semiconductor switching device is rendered conductive, the second semiconductor switching device is rendered nonconductive by the lower-frequency signal, and vice versa.
The DC-to-high-frequency converter is so controlled in synchronization with the first and second semiconductor switching devices as to provide the high-frequency signal having a value larger during a time when the second semiconductor switching device is conductive than during a time when the first semiconductor switching device is conductive. The control of the DC-to-high-frequency converter may be done by, for example, changing the value of a reference signal for feedback control of the converter, in synchronization with the control of the first and second semiconductor switching devices.
It is preferable that the period during which the second semiconductor switching device is conductive be shorter than the period during which the first semiconductor switching device is conductive.
With the above-described arrangement, when the first semiconductor switching device is rendered conductive with the second semiconductor switching device being nonconductive, positive current is supplied to the plating load from the first output-side rectifier. On the other hand, when the second semiconductor switching device is rendered conductive with the first semiconductor switching device being nonconductive, negative current is supplied to the plating load from the second output-side rectifier. Since the DC-to-high-frequency converter is arranged to provide a larger high-frequency signal when the second semiconductor switching device is conductive, i.e. when the negative current is being supplied to the plating load, than when the first semiconductor switching device is conductive, i.e. when the positive current is being supplied to the plating load. Accordingly, the negative current has a larger value so as to perform uniform plating.
The power supply apparatus with this arrangement requires only one input-side rectifier which functions as a DC supply. In addition, this electroplating power supply apparatus requires only two semiconductor switching devices. Accordingly, the cost of the power supply apparatus can be low.
First and second reactors may be connected to the first and second semiconductor switching devices, respectively. The first and second reactors are wound on the same core so that the positive voltage applied to the plating load can increase when the second semiconductor switching device is nonconductive, and the negative voltage applied to the plating load can increase when the first semiconductor switching device is nonconductive. For example, the first reactor may be wound on the core in the direction opposite to the direction in which the second reactor is wound.
When the first and second reactors are used in the manner as stated above, during a time when the first output-side rectifier rectifies the high-frequency signal from the transformer to provide positive current to the plating load, the first reactor discharges additional positive current to the plating load. Similarly, during a time when the second output-side rectifier rectifies the high-frequency signal from the transformer to provide negative current to the plating load, the second reactor discharges additional negative current to the plating load. Accordingly, the conversion between the positive current and the negative current supplied to the plating load can be performed at a high speed, which promotes the uniform plating.
Charge storage means may be charged when current is flowing in the load. A capacitor may be used as the charge storage means, or a snubber circuit associated with the semiconductor switching device may be arranged to function additionally as the charge storage means. When one of the first and second semiconductor switching devices is rendered conductive with the other being in the nonconductive state, discharging means causes the charge storage means to discharge in such a manner that discharge current of the same polarity as the current currently flowing into the plating load flows. The discharging means may be, for example, a switch connected between the charge storage means and the plating load.
With this arrangement, a charge stored in the charge storage means when current is supplied to the plating load is discharged when the polarity of the current flowing in the plating load is reversed. The charge is discharged in the polarity after the polarity reversal. Accordingly, the current to the plating load can be reversed from the positive to negative polarity or from the negative to positive polarity so that the thickness of the resulting plated layer can be uniform.
The DC-to-high-frequency converter may include a converting semiconductor switching device and control means for ON-OFF controlling the converting semiconductor switching device. The control means provides a control signal to ON-OFF control the converting semiconductor switching device in such a manner that the difference of the positive current flowing through the plating load from a positive current reference value set for the positive current can become zero, and the difference of the negative current flowing through the plating load from a negative current reference value set for the negative current can become zero. Sample and hold means is provided for sampling and holding the control signal provided by the converting semiconductor switching device control means when the negative current flows into the plating load, and the sampled and held control signal is applied to the converting semiconductor switching device when the current flowing through the load switches from the positive to negative current.
The reason for the use of the sample and hold means is as follows. The DC-to-high-frequency converter is feedback controlled. However, when the current to the plating load is switched from, for example, positive to negative polarity, the negative current cannot change to the value corresponding to the negative current reference value simultaneously with the polarity switching since the positive current reference value and the negative current reference value differ from each other. To avoid this problem, the control signal produced when the negative current is supplied is sampled and held, and the sampled and held control signal is applied to the converting semiconductor switching device of the DC-to-high-frequency converter when the current is switched to the negative one so that the negative current can be instantaneously changed to the one corresponding to the negative current reference value. This results in uniform thickness of the plated layer.
Detecting means for detecting when the load is opened may be used together with the first and second reactors. When the detecting means detects the open-circuiting of the load, that one of the first and second reactors through which current was flowing before such detection is short-circuited by a short-circuiting semiconductor switching device.
Open-circuiting of the load for any reason causes the current flowing currently through the first or second reactor becomes zero, which, in turn, causes a large voltage generated across the first or second reactor the current flowing through which has become zero. Application of such large voltage to the first or second semiconductor switching device may cause damage to that semiconductor switching device. To avoid it, when the load is open-circuited, the reactor through which current is flowing when the load is opened is short-circuited to thereby prevent the voltage generated across it from being applied to the first or second semiconductor switching device, to thereby protect the semiconductor switching device.
The DC-to-high-frequency converter may be formed of two inverters connected in series between the output terminals of the input-side rectifier. The two inverter configuration may be used when the commercial AC power supply supplies an AC voltage to the input-side rectifier which outputs an output voltage twice the voltage each inverter can bear.
With this arrangement, semiconductor devices which can withstand a lower voltage than the DC voltage produced from the input commercial AC voltage can be used as the semiconductor switching devices of the inverters. In other words, inexpensive semiconductor devices can be used, which, in turn, can reduce the cost of the power supply apparatus.